1. Field of the Invention
The present invention relates to radial flow lasers and more particularly to optical resonators capable of accommodating a large volume of gain medium and providing an output beam having good optical qualities.
2. Description of the Prior Art
Laser systems which provide output beams at very high power levels, such as those made possible with combustion driven chemical lasers are subject to various restraints which limit the actual output power. One of these limitations is due to a phenomenon which is commonly referred to as superfluorescence, a condition under which a gain medium having a population inversion produces spurious beams of laser radiation without an interaction with any optical components of the resonator. These spurious beams are undesirable because they deplete the population inversion which would otherwise be available for controlled stimulated emission from the working medium.
One of the concepts advanced to obtain output beams having high power levels without incurring superfluorescence effects is disclosed by Freiberg et al. in the U.S. Pat. No. 3,969,687 entitled "Standing Wave Unstable Resonators for Radial Flow Lasers" filed on Apr. 14, 1975 and held with the present application by a common assignee. The unstable resonator is formed having a centerline axis with a gain region in the configuration of a thin wall cylinder and disposed symmetrically about the centerline axis and between the optical components defining the unstable resonator. A cylindrical gas source region is disposed adjacent to the interior of the gain region and symmetric about the centerline axis. The system optics form an unstable resonator and the output beam is annular in cross section. The resonator is an unstable standing wave positive branch confocal device in which toroidal and annular mirrors are utilized. High optical quality of the output beam results from the effective discrimination against the high order transverse modes which is provided by the region of common resonance dominated by the diffraction cross coupling of the device.
Another concept advanced to provide an output beam with enhanced power, energy distribution and optical characteristics is disclosed by Chenausky et al. in the U.S. Pat. No. 3,921,096 entitled "Unstable Split Mode Laser Resonator" filed Dec. 16, 1974 and held with the present application by a common assignee. The unstable split mode resonator is of complex design utilizing two separate volumes of gain medium, each of which has dimensions no greater than the limitations imposed by superfluorescence. The phase of the beam in each of the regions becomes locked to the phase of the beam from the other region by an area of common resonance. The active gain medium is disposed between the optical components forming the unstable resonator and the resulting output beam has an annular cross section.
Another approach to increasing the output power involves arranging the gain medium in the form of a cylindrical sheath such as that produced by a radial flow configuration so that a relatively large volume can be handled without exceeding the superfluorescence length limitations. The sheath is positioned within a relatively simple unstable resonator comprising an annular convex toroidal surface at one end of the cylindrical sheath and an annular concave toroidal mirror at the other end of the cylindrical sheath. The utility of such a resonator configuration is severely limited by the poor optical quality of the laser beam which results therefrom. An unstable resonator arranged in such a cylindrical geometry has a high Fresnel number which is defined as the square of the outer diameter of the cylindrical sheath of the active medium divided by four times the product of the laser wavelength and the length of the cylinder. The high Fresnel number is an indication of very little coupling of the annular output beam phase front around the circumference of the cavity as the beam propagates between the mirrors and through the circumferentially oriented gain medium. As a consequence, such a resonator displays very poor mode discrimination and is prone to support high azimuthal modes, which have far field energy distribution exhibiting a minimum on the optical axis and departing significantly from the diffraction limited operation.
Present resonators for use with annular gain configurations in chemical lasers are of complex design, requiring the use of aspheric and conical optical components which have extremely difficult manufacturing problems and often produce annular output beams. Designs which use simple mirrors and produce an output beam having a continuous cross section, and lowest order mode are desirable.